1. Field of the Invention
This invention relates to a circuit for reverse phase-controlled dimming of an a.c. load.
2. Description of the Related Art
Phase-controlled dimming is accomplished by switching a.c. power to the load on and off during each half cycle. The amount of dimming is determined by the ratio of "on" to "off" time. In conventional phase-controlled dimming, the power is off at the beginning of each half cycle (i.e., at the zero crossing) and turns on later during the half cycle. In reverse phase-controlled (RPC) dimming, the power to the load is switched on at or near the zero crossing and is switched off later in each half cycle.
RPC dimming of gas discharge lamps was disclosed in U.S. Pat. No. 4,090,099, issued Jul. 4, 1978, to Grudelbach. (See also Pat. No. 4,350,935, issued Sep. 11, 1982, to Spira et al., and Pat. No. 4,527,099, issued Jul. 2, 1985, to Capewell et al.) The circuit disclosed in the Grudelbach patent includes a capacitor (C.sub.1 in FIG. 4) from load hot to neutral. The circuit serves to dissipate the energy stored in the inductors that serve as ballast for the gas discharge lamps. Incandescent lamps have no ballast, and energy stored in inductors is not a concern with these lamps.
RPC dimming of incandescent lighting was disclosed by R. M. Burkhart et al., IEEE Trans. Ind. Appl. vol. 1A-15, PP. 579-582, September/October, 1979 (see also IEEE Trans. Ind. Appl. vol. 1A-8, pp. 84-88 January/February, 1972). The circuit disclosed by Burkhart et al. includes a filter, or snubber, capacitor (C.sub.3 in FIG. 2 of the 1979 paper) connected from line hot to load hot. However, as discussed in the 1979 paper, pp. 581, 582, a single capacitor value is not satisfactory over a large range of loads, and the capacitance must be varied to accommodate changes in the load.
An approach to the load dependence of RPC dimming was disclosed by C. F. Christiansen and M. Benedetti, IEEE Trans. Ind. Appl. vol. 1A-19, May/June 1983, pp. 323-327. They corrected the load dependence of an RPC incandescent dimming circuit by switching using a power field-effect transistor (FET) in the active region. The falloff time when the switch opens can be controlled by circuitry that does not depend on the load. However, because the FET is in the active region, it dissipates power, which is undesirable for a number of reasons. The power dissipated in the switch constitutes wasted power that is costly, that speeds the rate of thermal degradation of the product elements, and that requires a larger dimmer, in order to dissipate the additional heat safely.
Bloomer has disclosed RPC dimming of a.c. loads, including incandescent lamps. (See U.S. Pat. Nos. 4,528,494 and 4,540,893). His switching is accomplished using FETs in the active region, which causes excessive power dissipation. He has also disclosed a protection circuit for controlled-conduction circuits (U.S. Pat. No. 4,547,828). However, he requires complex circuits that include direct line voltage input to the control section, which is undesirable, particularly when a capacitive load Is being controlled. In that case, the switch may be turned on when there is a voltage across the load capacitor, which can cause high surge currents, electromagnetic interference, and lamp buzzing.